Showing papers by "Kwang S. Kim published in 2018"

Multicomponent electrocatalyst with ultralow Pt loading and high hydrogen evolution activity

Abstract: Platinum is the most effective electrocatalyst for the hydrogen evolution reaction in acidic solutions, but its high cost limits its wide application. Therefore, it is desirable to design catalysts that only require minimal amounts of Pt to function, but that are still highly active. Here we report hydrogen production in acidic water using a multicomponent catalyst with an ultralow Pt loading (1.4 μg per electrode area (cm2)) supported on melamine-derived graphitic tubes (GTs) that encapsulate a FeCo alloy and have Cu deposited on the inside tube walls. With a 1/80th Pt loading of a commercial 20% Pt/C catalyst, in 0.5 M H2SO4 the catalyst achieves a current density of 10 mA cm−2 at an overpotential of 18 mV, and shows a turnover frequency of 7.22 s−1 (96 times higher than that of the Pt/C catalyst) and long-term durability (10,000 cycles). We propose that a synergistic effect between the Pt clusters and single Pt atoms embedded in the GTs enhances the catalytic activity. Although Pt is highly active for electrocatalytic production of H2 from water, its cost limits its wide application. Here, the authors prepare a high-performing catalyst that is supported on graphitic tubes, containing Fe, Co and Cu, and requires only a small amount of Pt.

Highly efficient organic photocatalysts discovered via a computer-aided-design strategy for visible-light-driven atom transfer radical polymerization

Abstract: Organocatalysed photoredox-mediated atom transfer radical polymerization (O-ATRP) is a very promising polymerization method as it eliminates concerns associated with transition-metal contamination of polymer products. However, reducing the amount of catalyst and expanding the monomer scope remain major challenges in O-ATRP. Herein, we report a systematic computer-aided-design strategy to identify powerful visible-light photoredox catalysts for O-ATRP. One of our discovered organic photoredox catalysts controls the polymerization of methyl methacrylate at sub-ppm catalyst loadings (0.5 ppm—a very meaningful amount enabling the direct use of polymers without a catalyst removal process); that is, 100–1,000 times lower loadings than other organic photoredox catalysts reported so far. Another organic photoredox catalyst with supra-reducing power in an excited state and high redox stability facilitates the challenging polymerization of the non-acrylic monomer styrene, which is not successful using existing photoredox catalysts. This work provides access to diverse challenging organic/polymer syntheses and makes O-ATRP viable for many industrial and biomedical applications. Organocatalysed photoredox-mediated atom transfer radical polymerization is a very promising method, although many challenges still lie ahead. Now, Kwon, Gierschner, Kim and co-workers present a computer-aided-design strategy to identify organic photoredox catalysts for this process. The success of the design strategy is demonstrated by polymerizations of methyl methacrylate and styrene.

Extremely stable graphene electrodes doped with macromolecular acid.

Abstract: Although conventional p-type doping using small molecules on graphene decreases its sheet resistance (Rsh), it increases after exposure to ambient conditions, and this problem has been considered as the biggest impediment to practical application of graphene electrodes. Here, we report an extremely stable graphene electrode doped with macromolecular acid (perfluorinated polymeric sulfonic acid (PFSA)) as a p-type dopant. The PFSA doping on graphene provides not only ultra-high ambient stability for a very long time (> 64 days) but also high chemical/thermal stability, which have been unattainable by doping with conventional small-molecules. PFSA doping also greatly increases the surface potential (~0.8 eV) of graphene, and reduces its Rsh by ~56%, which is very important for practical applications. High-efficiency phosphorescent organic light-emitting diodes are fabricated with the PFSA-doped graphene anode (~98.5 cd A-1 without out-coupling structures). This work lays a solid platform for practical application of thermally-/chemically-/air-stable graphene electrodes in various optoelectronic devices.

Water-Stable, Fluorescent Organic−Inorganic Hybrid and Fully Inorganic Perovskites

Abstract: Stabilization of perovskites without addition of foreign surface-passivating ligands in aqueous media is essential for their applications in optoelectronics, biomedical science, and catalysis. However, these materials instantly degrade in water due to its intrinsic ionic nature. By controlling the peripheral layer of octahedral perovskite geometry, we reproducibly synthesized a series of rod-shaped fluorescent hybrid perovskites in both acidic and basic media at ambient conditions in large scale without capping ligands. The band gap is tunable from the red to sky-blue region with sharp emission. The lead bromide perovskites are stable more than 6 months in water without structural change. Our simple synthetic route has resolved the longstanding problems for its practical application in aqueous environment.

Fulgides as Light-Driven Molecular Rotary Motors: Computational Design of a Prototype Compound.

Abstract: A new family of light-driven molecular rotary motors utilizing the fulgide motif is proposed and its prototype molecule is studied by quantum chemical calculations and nonadiabatic molecular dynamics simulations. The new motor performs pure unidirectional axial rotation of the rotor blade with high quantum efficiency (ϕ ∼ 0.55–0.68) and ultrafast dynamics (⟨t⟩S1 ∼ 200–300 fs) of its successive photoisomerization steps. The photocyclization reaction typical of fulgide compounds is blocked by the design of the new motor and never occurred in the molecular dynamics simulations. The new motors can be synthesized from easily available precursors. In view of its remarkable photoisomerization ability, the new motor represents a prospective class of compounds for the use in nanosized molecular devices.

Turn-on and Turn-off Fluorescent Probes for Carbon Monoxide Detection and Blood Carboxyhemoglobin Determination.

Abstract: Water-soluble, carbazole-based two-photon excitable fluorescent probes MPVC-I (“turn-on”) and MPVC-II (“turn-off”) are rationally designed and synthesized for the selective monitoring of carbon monoxide (CO). Both probes can effectively measure carboxyhemoglobin (HbCO) in the blood of the animals exposed to a CO dose as low as 100 ppm for 10 min. The palladium catalyzed azidocarbonylation reaction was optimized to improve the sensing efficiency.

Rashba–Dresselhaus Effect in Inorganic/Organic Lead Iodide Perovskite Interfaces

Abstract: Despite the imperative importance in solar cell efficiency, the intriguing phenomena at the interface between perovskite solar cell and adjacent carrier transfer layers are hardly uncovered. Here we show that PbI2/AI-terminated lead iodide perovskite (APbI3; A = Cs+/ methylammonium (MA)) interfaced with the charge transport medium of graphene or TiO2 exhibits a sizable/robust Rashba–Dresselhaus (RD) effect using density functional theory and ab initio molecular dynamics (AIMD) simulations above the cubic-phase temperature. At the PbI2-terminated graphene/CsPbI3(001) interface, ferroelectric distortion toward graphene facilitates an inversion breaking field. At the MAI-terminated TiO2/MAPbI3(001) interface, the enrooted alignment of MA+ toward TiO2 by short-strong hydrogen bonding and concomitant PbI3 distortion preserve the RD interactions even above 330 K. The robust RD effect at the interface even at high temperatures, unlike in bulk, changes the direct-type band to indirect-type to suppress recombinati...

La-doped BaSnO3 electron transport layer for perovskite solar cells

Abstract: Due to the photoinstability and large hysteresis of the TiO2 electron transport layer (ETL) in perovskite solar cells (PSCs), the search for novel electron transport materials has emerged. Herein, using first principles calculations, we unveiled the key factor for the ideal band alignment between La-doped BaSnO3 (LBSO) and methyl ammonium (MA) lead iodide perovskite (MAPbI3). The (PbI2-, MAI-terminated)CH3NH3PbI3/(SnO2-, BaO-terminated)LaxBa(1−x)SnO3 interface formed a stable “all-perovskite” heterostructure with large binding energy. The selective band alignment of the conduction band was easily manipulated by La-doping on the Ba site due to the band gap renormalization (or shrinkage) caused by doped electrons and La3+ dopant. In addition, the MAI-terminated MAPbI3/LBSO interface exhibited proton transfer (BaO-terminated) and strong hydrogen bonding (SnO2-terminated) between MA and oxygen anion. LBSO presenting high mobility, photostability, and structural stability would help develop the next generation ETL materials for PSCs.

Direct Nonadiabatic Dynamics by Mixed Quantum-Classical Formalism Connected with Ensemble Density Functional Theory Method: Application to trans-Penta-2,4-dieniminium Cation

Abstract: In this work, a direct mixed quantum-classical dynamics approach is presented, which combines two new computational methodologies. The nuclear dynamics is solved by the decoherence-induced surface hopping based on the exact factorization (DISH-XF) method, which is derived from the exact factorization of the electronic-nuclear wave function and correctly describes quantum decoherence phenomena. The state-interaction state-averaged spin-restricted ensemble-referenced Kohn-Sham (SI-SA-REKS, or SSR, for brevity) electronic structure method is based on ensemble density functional theory (eDFT) and provides correct description of real crossings between the ground and excited Born–Oppenheimer electronic states. The new combined approach has been applied to the excited-state nonadiabatic dynamics of the trans-penta-2,4-dieniminium cation (PSB3). The predicted S1 lifetime of trans-PSB3, τ = 99 ± 51 fs, and the quantum yield of the cis conformation, ϕ = 0.63, agree with the results obtained previously in nonadiabat...

An ultra-sensitive, flexible and transparent gas detection film based on well-ordered flat polypyrrole on single-layered graphene

Abstract: Real-time gas sensors that are ultra-sensitive, highly selective, and capable of fast response/recovery are highly in demand for environmental and health monitoring; however, advances in such gas sensors have been limited. Here we report a flexible and transparent sub-ppb gas detection film fabricated by in situ electrochemical oxidative polymerization on single-layer graphene (SLG). As polypyrrole molecules are uniformly formed with flat orientations on SLG, the resulting sensor film exhibits excellent sensitivity and selectivity with nearly perfect reversibility without heat treatment or light irradiation. The sensing mechanism is well explained by molecular interactions between the sensing material and the gas molecular species. The sensor exhibits a detection limit as low as 0.03 ppb NO2 and 0.04 ppb NH3 at room temperature and operates well, even at 70 °C, showing feasibility for wearable applications in high-precision gas sensing.

A highly hydrophobic fluorographene-based system as an interlayer for electron transport in organic–inorganic perovskite solar cells

Abstract: Degradation of perovskite halide materials under humid conditions is one of the major hurdles in the commercialization of organic–inorganic perovskite solar cells. Herein, we studied the interface between highly hydrophobic fluorographene (FGr) and cubic methylammonium lead iodide (MAPbI3, MA: CH3–NH3) by employing density functional theory (DFT)-based simulations. We demonstrate that the adsorption of FGr on MAPbI3 results in the formation of a stable interface with appreciable binding energy (∼0.4 eV per Pb atom). Thorough assessment of energy-level alignment indicates that the FGr/MAPbI3 interface has desirable properties with regard to the electron transfer (hole blockage) process. These results underscore the potential of using FGr as an interlayer for electron transport between a perovskite layer and an electron transfer medium (such as TiO2) as well as a moisture blocker for achieving high perovskite stability by perfect waterproofing. The future research study towards the integration of hydrophobic FGr or electronically optimized partially fluorinated graphene-based systems within perovskite halide photovoltaic devices may pave the way for stable and efficient solar cell technologies.

Rashba-Dresselhaus Effect in Inorganic/Organic Lead Iodide Perovskite Interfaces

Abstract: Despite the imperative importance in solar-cell efficiency, the intriguing phenomena at the interface between perovskite solar-cell and adjacent carrier transfer layers are hardly uncovered. Here we show that PbI$_2$/AI-terminated lead-iodide-perovskite (APbI$_3$; A=Cs$^+$/ methylammonium(MA)) interfaced with the charge transport medium of graphene or TiO2 exhibits the sizable/robust Rashba-Dresselhaus (RD) effect using density-functional-theory and ab initio molecular dynamics (AIMD) simulations above cubic-phase temperature. At the PbI$_2$-terminated graphene/CsPbI3(001) interface, ferroelectric distortion towards graphene facilitates an inversion breaking field. At the MAI-terminated TiO$_2$/MAPbI$_3$(001) interface, the enrooted alignment of MA$^+$ towards TiO$_2$ by short-strong hydrogen-bonding and the concomitant PbI$_3$ distortion preserve the RD interactions even above 330 K. The robust RD effect at the interface even at high temperatures, unlike in bulk, changes the direct-type band to the indirect to suppress recombination of electron and hole, thereby letting these accumulated carriers overcome the potential barrier between perovskite and charge transfer materials, which promotes the solar-cell efficiency.

Intramolecular deformation of zeotype-borogermanate toward a three-dimensional porous germanium anode for high-rate lithium storage

Abstract: We demonstrate a new class of synthetic process for three-dimensional porous Ge materials (3D-pGe). Starting from zeotype-borogermanate microcubes, the 3D-pGe sample was synthesized through a thermal deformation of artificial Ge-rich zeolite, etching, and subsequent hydrogen reduction. After the synthesis, the resultant byproducts were simply removed by warm water instead of a harmful etchant such as hydrofluoric acid. Benefiting from the structural advantages with meso/macro porosity in the overall framework, the as-prepared 3D-pGe exhibits good electrochemical properties as anode materials for lithium-ion batteries with a high capacity (770 mA h g−1), cycling stability (capacity retention over 83%) after 250 cycles at 1C, and excellent rate capability (32% for 10C with respect to C/5) as well as pseudocapacitive contribution by surface-controlled reaction. This study paves the way to a new synthesis strategy of 3D porous Ge anode materials from zeolite for large-scale energy storage applications.

Organic cation steered interfacial electron transfer within organic–inorganic perovskite solar cells

Abstract: Methylammonium lead-iodide (MAPbI3, MA: CH3–NH3) interfaced with rutile TiO2 is widely used in photovoltaic devices. These devices utilize the electron transfer from MAPbI3 to TiO2, which may not be explained solely by the band structures of the two bulk materials. To elucidate the interface dynamics and its impact on the electron transfer process, we have studied the interfacial features of a TiO2/MAPbI3 system. First principles calculations and ab initio molecular dynamics simulations show that the rotational freedom of MA present within the bulk is considerably suppressed due to interaction of MA with the TiO2 substrate, highlighting orientationally ordered MA at the interface. The optimized interface structure shows the C–N axis of MA titled towards the TiO2 surface so as to maximize the interaction between N-attached H and underlying O. The very short O⋯H⋯N distance with very large hydrogen bonding energy identifies short strong hydrogen bonding (SSHB) as the origin of structural re-organization at the interface. As for the electronic structure, this proton sharing between MA and TiO2 has a critical impact on the energy level alignment at the interface, thus driving the electron transfer process from MA to TiO2. Indeed, significant reduction in the electron transfer barrier is observed for the energetically optimal interface configuration which promotes the electron transfer across the interface and photovoltaic properties.

Ambient-Stable Cubic-Phase Hybrid Perovskite Reaching the Shockley–Queisser Fill Factor Limit via Inorganic Additive-Assisted Process

Abstract: Additive-assisted organic–inorganic perovskite materials have attracted substantial attention as photovoltaic light absorbers which lead to outstanding power conversion efficiency. Here we report an easy and effective fabrication of cubic-phase perovskite with an inorganic molecule additive like hydrazinium chloride (N2H5Cl, to be denoted as HZCl). We predict that this inorganic cation of N2H5+, which can substitute for the organic A-site in the perovskite structure, can tune Frohlich polaron properties by controlling the interaction strength and the number of proton coordinations to halide. This prediction is experimentally demonstrated with an optimized perovskite device with 2% N2H5Cl additive, which exhibits an unprecedented 85% fill factor (FF) with the highest value close to the Shockley–Queisser limit. An extra power conversion efficiency (PCE) of 2.3% and a fill factor (FF) efficiency of 14% are boosted. These optimized performances by additive effects lead to a new approach based on the theoretic...

Observation of the possible chiral edge mode in Bi1-xSbx

Abstract: After the classification of topological states of matter has been clarified for non-interacting electron systems, the theoretical connection between gapless boundary modes and nontrivial bulk topological structures, and their evolutions as a function of dimensions are now well understood. However, such dimensional hierarchy has not been well established experimentally although some indirect evidences were reported, for example, such as the half-quantized Hall conductance via quantum Hall effect and extrapolation in the quantum-oscillation measurement. In this paper, we report the appearance of the possible chiral edge mode from the surface state of topological insulators under magnetic fields, confirming the dimensional hierarchy in three dimensional topological insulators. Applying laser pulses to the surface state of Bi1-xSbx, we find that the sign of voltage relaxation in one edge becomes opposite to that in the other edge only when magnetic fields are applied to the topological insulating phase. We show that this sign difference originates from the chirality of edge states, based on coupled time-dependent Poisson and Boltzmann equations.

Anisotropic and amphoteric characteristics of diverse carbenes

Abstract: Despite its key importance in carbene chemistry, the amphoteric (i.e., both nucleophilic and electrophilic) behavior of the divalent carbon atom (:C) in carbenes is not well understood. The electrostatic potential (EP) around :C is often incorrectly described by simple isotropic atomic charges (particularly, as in singlet CF2); therefore, it should be described by the multipole model, which can illustrate both negative and positive EPs, favoring the positively and negatively charged species that are often present around :C. This amphotericity is much stronger in the singlet state, which has more conspicuous anisotropic charge distribution than the triplet state; this is validated by the complexation structures of carbenes interacting with Na+, Cl−, H2O, and Ag+. From the study of diverse carbenes [including CH2, CLi2/CNa2, CBe2/CMg2, CF2/CCl2, C(BH2)2/C(AlH2)2, C(CH3)2/C(SiH3)2, C(NH2)2/C(PH2)2, cyclic systems of C(CH2)2/C(CH)2, C(BHCH)2, C(CH2CH)2/C(CHCH)2, and C(NHCH)2/C(NCH)2], we elucidate the relationships between the electron configurations, electron accepting/donating strengths of atoms attached to :C, π conjugation, singlet–triplet energy gaps, anisotropic hard wall radii, anisotropic electrostatic potentials, and amphotericities of carbenes, which are vital to carbene chemistry. The (σ2, π2 or σπ) electronic configuration associated with :C on the :CA2 plane (where A is an adjacent atom) in singlet and triplet carbenes largely governs the amphoteric behaviors along the :C tip and :C face-on directions. The :C tip and :C face-on sites of σ2 singlet carbenes tend to show negative and positive EPs, favoring nucleophiles and electrophiles, respectively; meanwhile, those of π2 singlet carbenes, such as very highly π-conjugated 5-membered cyclic C(NCH)2, tend to show the opposite behavior. Open-shell σπ singlet (such as highly π-conjugated 5-membered cyclic C(CHCH)2) and triplet carbenes show less anisotropic and amphoteric behaviors.

Spectromicroscopic observation of a live single cell in a biocompatible liquid-enclosing graphene system

Abstract: On-the-spot visualization of biochemical responses of intact live cells is vital for a clear understanding of cell biology. The main obstacles for instant visualization of biochemical responses of living cells arise from the lack of a sophisticated detecting technique which can simultaneously provide chemical analysis tools and the biocompatible wet conditions. Here we introduce scanning transmission X-ray microscopy (STXM) combined with a liquid-enclosing graphene system (LGS), offering biocompatible conditions and improved X-ray absorption spectra to probe the chemical responses of live cells under wet conditions. This set-up enables us to probe a subtle change in absorption spectra depending on the oxidation state of a miniscule amount of oxygen in the functional groups present in each cell and its surroundings containing a minimal amount of liquid water. As an example of in situ biochemical responses of wet cells, chemical responses of a single Colo 205 cell are visualized and analyzed using X-ray absorption near the oxygen K-edge. This spectromicroscopic method using LGS can be applied to diverse biological samples under wet conditions for the analysis of their biochemical responses.

Sulfur-vacancy-dependent geometric and electronic structure of bismuth adsorbed on Mo S 2

Abstract: Through Bi deposition on the single-crystalline $\mathrm{Mo}{\mathrm{S}}_{2}$ surface, we find that the density of the sulfur vacancy is a critical parameter for the growth of the crystalline Bi overlayer or cluster at room temperature. Also, the $\mathrm{Mo}{\mathrm{S}}_{2}$ band structure is significantly modified near \ensuremath{\Gamma} due to the orbital hybridization with an adsorbed Bi monolayer. Our experimental observations and analysis in combination with density functional theory calculation suggest the importance of controlling the sulfur vacancy concentration in realizing an exotic quantum phase based on the van der Waals interface of Bi and $\mathrm{Mo}{\mathrm{S}}_{2}$.

Observation of the possible chiral edge mode in Bi1−x Sb x

Abstract: After the classification of topological states of matter has been clarified for non-interacting electron systems, the theoretical connection between gapless boundary modes and nontrivial bulk topological structures, and their evolutions as a function of dimensions are now well understood. However, such dimensional hierarchy has not been well established experimentally although some indirect evidences were reported, for example, such as the half-quantized Hall conductance via quantum Hall effect and extrapolation in the quantum-oscillation measurement. In this paper, we report the appearance of the possible chiral edge mode from the surface state of topological insulators under magnetic fields, confirming the dimensional hierarchy in three-dimensional topological insulators. Applying laser pulses to the surface state of Bi1−x Sb x , we find that the sign of voltage relaxation in one edge becomes opposite to that in the other edge only when magnetic fields are applied to the topological insulating phase. We show that this sign difference originates from the chirality of edge states, based on coupled time-dependent Poisson and Boltzmann equations.

Space Reduction in Matrix Product State

Abstract: We reconstruct a matrix product state (MPS) in reduced spaces using density matrix. This scheme applies to a MPS built on a blocked quantum lattice. Each block contains $N$ physical sites that have a local space of rank $R$. The simulation in the original spaces of rank $R^N$ is used to construct density matrices for every block. They are diagonalized and only the eigenvectors corresponding to significant diagonal elements are used to transform the original spaces to smaller ones and to reconstruct the MPS in those smaller spaces accordingly. Simulations in the reduced spaces are used to reliably extrapolate the result in unreduced spaces. Moreover, to obtain a required accuracy, the ratio of the reduced space rank over the original decreases quickly with $N$. The reduced space has a saturated rank to obtain a demanded accuracy when $N\rightarrow \infty$.

La-Doped BaSnO$_3$ Electron Transport Layer for Perovskite Solar Cells.

Abstract: Due to the photo-instability and hysteresis of TiO$_2$ electron transport layer (ETL) in perovskite solar cells (PSCs), novel electron transport materials are highly demanded. Here, we show ideal band alignment between La-doped BaSnO$_3$ (LBSO) and methyl ammonium (MA) lead iodide perovskite (MAPbI$_3$). The CH$_3$NH$_3$PbI$_3$/La$_x$Ba$_{(1-x)}$SnO$_3$ interface forms a stable all-perovskite heterostructure. The selective band alignment is manipulated with band gap renormalization by La-doping on the Ba site. LBSO shows high mobility, photo-stability, and structural stability, promising the next generation ETL materials.